This invention relates to wood stoves. More particularly, this invention relates to an air-tight wood stove with a forced air heat transfer system and means for automatically preventing overheating of the stove if the forced air system is not operative.
Petroleum products have constituted, either directly or indirectly, the principal home heating fuel for at least the last three decades. In recent years restrictions on the availability of petroleum and substantial price increases therefor have fostered a renewed interest in the use of wood stoves for home heating. Wood is a highly advantageous fuel in most areas where it is available and the supply is infinitely renewable. Moreover, in an efficient air-tight stove, the use of wood as a heating fuel is relatively economical.
In response to this interest, a number of air-tight wood stoves have come on the market. Typically, such stoves are designed with tightly sealed fireboxes so that substantially all combustion air for the fire must enter through designated air inlets. By proper adjustment of the air inlets, a controlled burning of the wood fuel in the firebox can be achieved, so that the fire may be maintained even overnight before additional fuel must be charged to the firebox. Large numbers of such stoves have been sold and installed. Unfortunately, in recent years there has been a high incidence of fires in homes equipped with wood stoves, and the general safety record of wood stoves has been poor. Individual owners of stoves often have been unaware of the possible hazards and many serious fires have resulted from the overheating of adjacent combustible walls, floors or furniture. Fires have also arisen from overheating of the stoves themselves which causes cracking and warping of the metal parts.
Concern over the safety of wood stoves has led to the development of safety standards. For example, local jurisdictions may require that a stove must meet Underwriters Laboratories or equivalent test standards which for a cast iron stove with structural walls 3/16" thick require the maximum sustained stove wall temperature under test firing conditions not to exceed 900.degree. F. Stove wall temperatures up to 1000.degree. F. and flue gas temperatures up to 1400.degree. F. may be permitted for periods of short duration. Such standards are not unduly restrictive, and it is regrettable that manufacturers have not been able to meet these safety requirements.
Energy transfer from a wood stove to the surrounding environment occurs primarily in two ways--by radiation and by convection. Radiant energy is transmitted directly from the stove to all surfaces "seen" by the stove. As these surfaces absorb the radiant energy, the surface temperatures are raised and the adjacent air is warmed by conduction. Convective heat transfer from the stove occurs because the air next to the stove is heated directly and then moves upward, to be replaced by more air which is subsequently heated by the hot stove. The amount of convective heat transfer is strongly dependent on the air velocity adjacent to the stove.
The greatest proportion of energy transferred from a wood stove is in the form of radiant energy and this proportion increases as the stove temperature increases. An exposed vertical hot surface at 400.degree. F. (in a room at 70.degree. F.) transfers about 1200 BTUs of energy per hour per square foot. Radiation accounts for approximately 800 BTU/hr ft.sup.2 (67%) while convection yields 400 BTU/hr ft.sup.2 (33%). At a surface temperature of 800.degree. F. radiation yields about 4200 BTU/hr ft.sup.2 (74%) while convection yields about 1100 BTU/hr ft.sup.2 (26%). At 1200.degree. F., the radiant heat transfer is 12800 BTU/hr ft.sup.2 (85%) and the convective heat transfer is 2270 BTU/hr ft.sup.2 (15%).
The intensity of radiated heat energy from the stove varies indirectly as the square of the distance from the stove. Thus, walls or furnishings which are near a hot stove receive large amounts of radiant energy and the wall surface temperature can be raised sufficiently above ignition temperature so that a fire may be started. To prevent such fires it is often required to position stoves substantial distances from surrounding walls and furnishings. Such measures are often inconvenient and wasteful of space and consequently many stove owners fail to observe safe spacings.
Another disadvantage of radiation heat transfer from a hot stove occurs when radiated energy strikes an outside wall of a structure. The increased wall temperature causes greater heat losses to the outside atmosphere.
Some stoves have been designed with shells surrounding the firebox through which air flows. Because of the double barrier, heat transfer by radiation is greatly reduced. The stove size and weight are increased by the shell, and the stove efficiency is usually reduced unless a large amount of stove surface area relative to firebox size is provided.
The efficiency of wood stoves is an important consideration. The overall efficiency depends on how completely the wood is burned, the amount of excess air used for the combustion and the heat transfer efficiency from the interior of the stove to the room. Firebox design, size and location of air inlets, type of wood, etc., determine the completeness of combustion. A well designed stove usually has almost complete combustion. The quantity of excess air admitted to the stove varies widely depending on the design of the stove and on whether the fuel firing rate is high or low. A large amount of excess air carries a significant fraction of the heat up the chimney, thus wasting energy. The heat transfer efficiency depends very strongly on the stove area available for heat transfer, the residence time of the flue gases in the stove and the temperatures of the fire, the flue gases and the stove walls. Thus, heat transfer efficiency at low firing rates is higher than at high firing rates. As the size of a stove increases, the problem of transferring heat from the stove becomes more difficult. The maximum heat producing capacity of a stove depends on the internal volume of the firebox among other factors. The heat transfer capacity depends on the surface area. Since the volume of a stove increases as the cube of its characteristic dimension and the surface area increases only as the square thereof, it can be seen that the maximum heat producing capacity increases more rapidly than the heat transfer capacity as the size of the stove increases. This leads either to higher stove and flue gas temperatures or to limitations on the combustion rate per unit of firebox volume to values below those of smaller stoves.
If the stove wall temperature can be decreased while maintaining the same rate of heat transfer to the room (by improving the convection heat transfer for example), then the heat transfer efficiency can be raised because more heat will be transferred from the fire and the flue gases to the cooler stove wall. The flue gases will leave the stove at a lower temperature and the stack heat loss will be less.